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4-3
Section
CPM2A/CPM2C Communications Functions
236
Frame Transmission and Reception
In Host Link communications, the host computer ordinarily has the transmission
right first and initiates the communications. The CPM2A/CPM2C then automati-
cally sends a response.
Commands and responses are exchanged in the order shown in the illustration
below. The block of data transferred in a single transmission is called a “frame.”
A single frame is configured of a maximum of 131 characters of data. The right to
send a frame is called the “transmission right.” The Unit that has the transmis-
sion right is the one that can send a frame at any given time. The transmission
right is traded back and forth between the host computer and the CPM2A/
CPM2C each time a frame is transmitted. The transmission right is passed from
the transmitting Unit to the receiving Unit when either a terminator (the code that
marks the end of a command or response) or a delimiter (the code that sets
frames apart) is received.
Terminator
FCS
Text
Text
End code
Header code
Unit no.
Unit no.
Header code
FCS
Terminator
Frame (response)
Frame (command)
Next frame transmission
enabled (i.e., transmission
right transferred)
Text
Unit no.
Header code
FCS
Terminator
Frame (command)
Terminator
FCS
Text
End code
Header code
Unit no.
Frame (response)
Host
computer
CPM2A/CPM2C
Data transmissions from the PC to the host computer can be initiated by the CPU
Unit using TXD(48).
Text
Unit no.
Header code
FCS
Terminator
Frame (response)
Host
computer
CPM2A/CPM2C
There is no response
from the host computer
Slave-initiated
Communications
Содержание SRM1 - PROGRAMING 02-2001
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